The invention is based on an interferometric measuring device for measuring the shape of a surface of an object, with a radiation source that emits a short coherence radiation, a beam splitter for producing an object beam, which is guided along an object light path to the object, and a reference beam, which is guided along a reference light path to a reflective reference plane, and with an image recorder that records the radiation, which is reflected by the object and the reference plane and is brought into interference, and supplies it to an evaluation device to determine the surface shape.
An interferometric measuring device of this kind has been disclosed by DE 41 08 944 A1. In this known interferometric measuring device, which is based on the measuring principle of so-called white light interferometry or short coherence interferometry, a radiation source emits short coherence radiation, which a beam splitter splits into an object beam that illuminates a measurement object and a reference beam that illuminates a reflective reference plane in the form of a reference mirror. In order to scan the object surface in the depth direction, the reference mirror is moved in the direction of the optical axis of the reference light path by means of a piezoelectric actuating element. When the object light path and the reference light path coincide, then in the range of the coherence length, there is a maximum of interference contrast, which can be detected by means of a photoelectric image recorder and a subsequent evaluation device and is evaluated on the basis of the known deflection position of the reference mirror in order to determine the contour of the object surface.
Other interferometric measuring devices or interferometric measuring methods of this kind based on white light interferometry are given in “Surface Profiling by Analysis of White-Light Interferograms in the Spatial Frequency Domain” by P. de Groot and L. Deck, in the Journal of Modern Optics, Vol. 42, No. 2, pp. 389-401, 1995 and in “Endoskopisches 3-D-Formmesssytem” [Endoscopic 3-D Shape-Measuring System] by T. Maack, G. Notni, W. Schreiber, and W.-D. Prenzel, in the Jahrbuch für Optik und Feinmechanik [Annual of Optics and Fine Mechanics], Ed. W.-D. Prenzel, Verlag Schiele und Schoen, Berlin, pp. 231-240, 1998.
With the above-mentioned interferometric measuring devices and measuring methods, it is difficult to execute measurements of different locations, in particular ones that are difficult to access, e.g. in deep cavities or narrow channels, with a sufficient degree of lateral resolution. In order to eliminate this problem, the (unpublished) German Patent Application No. 199 48 813 proposes generating at least one intermediate image in the arm of the object light path, which achieves a greater lateral resolution even in a narrow cavity or narrow channel. On the other hand, enlarging the numerical aperture shortens the depth of focus and also, a scanning of a surface region whose normal (viewing direction) is oblique to the axis of the image-generating device of the object light path causes problems in the scanning in the depth direction.
The problem of maintaining the depth of focus range in depth scanning can be avoided in that instead of moving the reference plane or the reference mirror that represents it, the reference light path is kept fixed and the object light path is changed. This can once again take place in two different ways, i.e. on the one hand, by moving the object itself in the depth direction or on the other hand, by moving the interferometric part of the measuring device in relation to the object. These kinds of changes to the object light path as steps taken for depth scanning in white light interferometry are in fact known in and of themselves, for example from the above-mentioned journal articles, but are technically difficult to achieve, particularly in the manufacture of their parts.